Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
In recent years there has been a marked increase in the number and variety of micro-organisms which have been identified as particularly problematic in hospital environments. Increased labour costs have reduced the frequency and intensity with which walls, floors, ceilings and other surfaces are scrubbed with disinfectants and/or sterilizing agents. Although this treatment still occurs to a somewhat restricted extent in operating theaters, it is more usual for such treatment to be limited to surfaces within a 2 meter radius of an operating table. Scrub-down treatment is rarely extended these days to general wards or public areas. Moreover there are many surfaces within scrubbed areas such as the interior of locks, the guards covering castor wheels, the underside of doors, the occluded surface of hinges which are not satisfactorily treated by scrub-down and may harbour organisms. In addition in some operating theaters, electronic equipment such as computers and the like include fans which blow cooling air through equipment and carry particles which may harbour microorganisms into equipment housing where microorganisms may multiply on interior surfaces.
It has been proposed to disinfect both small and large spaces and enclosing surfaces with biocidal gases such as ozone or chlorine dioxide which are oxidative or corrosive and toxic, or with biocidal gases such as ethylene oxide and aldehydes, such as glutaraldehyde or formaldehyde. However such biocides are all extremely toxic, hazardous to use and may leave potentially harmful residues on surfaces. Steam is sometimes used but is hazardous to the operator and detrimental to many materials and much equipment because of the high temperatures involved, and as it leaves dense moisture on the surface it may lead to rusting or deterioration.
In recent years the use of hydrogen peroxide or peracetic acid as a disinfectant has become greatly preferred. Prior to the 1990s these peroxides were considered too unstable and hazardous to allow fumigation with the vapours. Nevertheless, various proposals have been made to use hydrogen peroxide in the vapour phase for disinfection/sterilization. Mostly vapour phase systems have been applicable to small volume chambers such as sterilizers that can be evacuated since the vapours are more effective at very low pressures or as plasmas (e.g. Schmidt U.S. Pat. No. 4,863,688). At the end of the treatment cycle, residual hydrogen peroxide vapour is pumped out by the vacuum pump and exhausted to atmosphere directly or via a catalytic destroyer which decomposes any residual peroxide vapour into harmless oxygen and water. In older vapour based instrument reprocessing systems it was proposed to remove surface contamination by use of rinse water, which risked water damage and introduced a drying problem since drying is both energy intensive and of long duration. Disadvantageously, water rinsing imposes a need for a water supply and drainage system which is a major disadvantage in some locations.
Peroxide vapours have also been proposed for use at atmospheric pressure but in that case longer treatment times are generally involved than in vacuum systems and efficacy against bacterial spores has been shown to be limited. After treatment in small scale peroxide vapour systems, air is circulated through the chamber and any residual peroxide is either flushed directly into the atmosphere through a HEPA-filter, or is flushed into the atmosphere via a catalytic destructor so that the peroxide is catalysed to oxygen and water prior to disposal. In some recirculating systems the flow may be diverted after the treatment and recirculated by an air pump though a catalytic destroyer placed in parallel with the treatment circuit until peroxide is eliminated (e.g. Hill U.S. Pat. No. 7,238,330; U.S. Pat. No. 6,953,549).
Others have proposed to use peroxide aerosols (rather than vapour) as the biocidal agent for sterilization/disinfection of small chambers. Aerosols have a number of major advantages over vapour including that much higher concentration density of active species is obtainable at atmospheric pressure and the need for costly vacuum equipment is eliminated. In some such cases the aerosol flow may be diverted through a catalytic destructor after the treatment cycle is completed to remove any peroxide residues (see applicant's commonly owned WO 2007/014436 and WO2007/014438).
Both peroxide vapour and peroxide aerosol systems have also been proposed for disinfection/sterilization or decontamination of larger spaces. In such case it mostly appears that any residue is merely flushed into the external environment. Ronlan in U.S. Pat. No. 6,500,465 proposed the use of a thermo fogger (pulse jet fogger) to provide a high density aerosol (aerosol droplet diameter less than 50 microns) of peracetic acid or hydrogen peroxide suitable for disinfecting at 100% relative humidity, but does not discuss peroxide disposal. Adiga U.S. Pat. No. 7,326,382 discloses something similar but likewise does not discuss peroxide disposal. Applicant's commonly owned WO2007/014437 (Erickson) and WO2007/014435 (Berentsveig et al.) have also used aerosols for large scale disinfection purpose. Erickson envisaged a portable catalytic destructor unit that could be moved from chamber to chamber and used to remove excess peroxide from a treated chamber, saying “The reservoir, nebuliser, fan, and heater may be combined in a portable unit which can be moved from chamber to chamber, and if desired a separate air drying or air conditioning system may be made portable for use in the same chamber as the nebuliser or may be combined with the nebuliser unit”, while Berentsveig vented residual peroxide directly or optionally via a catalytic destructor. Residual peracetic acid suffers from the additional disadvantage of unacceptable odour
While such stable mists of aqueous biocides, preferably hydrogen peroxide, can be employed at atmospheric pressure and above and avoid the need for vacuum equipment and are more easily adaptable to disinfection or fumigation of very large spaces, elimination of residual hydrogen peroxide remains a significant problem.
For example in sterilizing food containers with peroxide, even trace amounts of hydrogen peroxide can affect the flavor of the product or result in other undesirable changes, such as a change in the color of the product. Food packaging regulations now limit hydrogen peroxide residues on containers to a maximum of 0.5 ppm in the United States. Surface residues in operating theaters or on surgical instruments should be below 1 ppm. To achieve such levels by blowing or sucking air even though small chamber volumes for sterilizing instruments or the like can add significantly to process times, but to do so through a room size volume of 50-100 cu meters requires huge capital equipment and energy costs, especially when the incoming air needs also to be HEPA-filtered to maintain sterility. The volume of air in the room needs to be replaced more than ten times. The removal step thus adds greatly to treatment times because the residual balance of peroxide reduces asymptotically. A significant time (hours in a large building) is required to remove the last few ppm of sterilant and the space may not be safely re-habitable until this step is completed. For example a Steris VHP1000 vapour system takes up to 6 hrs to treat a 56 m3 room. The larger the volume of space treated the more difficult the removal problem becomes as the time for removal increases asymptotically.
In recent years particular attention has been paid to spaces which have become infected as a result of acts of war or terrorism. For example in the US a number of federal buildings were thought to have been contaminated by Anthrax spores. These were treated with chlorine gas which was very damaging to the building but in addition required a long time before the chlorine could be effectively removed to a level at which the building could be made habitable. In any disinfection and or sterilization method, it is important that the overall duration of the process including both treatment with biocide and removal of biocide to a point where the space can be safely occupied be minimized.
Chemical disinfection agents such as hypochlorite solutions have in the past been proposed for deactivating biological warfare agents but such biocidal agents are themselves harmful to personnel and equipment due to corrosiveness and toxicity.
In summary peroxy compounds such as hydrogen peroxide, hydrogen peroxide complexes, and peracetic acid are preferred as agents for disinfection/sterilization of surfaces, (eg medical instruments and food processing machinery and operating tables); for disinfection/sterilization of small and large chambers and of large spaces, as well as for deactivation of biological warfare agents. Peroxide has been used both as a vapour and as an aerosol for this purpose. Although disinfection/sterilization can be achieved in very short times, removal of residual peroxide down to safe levels of below 1 ppm, preferably below 0.5 ppm is a major problem, taking an unacceptably long time, and being very costly in terms of required equipment and energy consumption. The time problem increases greatly for large volume spaces. Many other biocides also have residual toxicity or corrosive properties which renders their use impractical.